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Delivery of complex organic compounds from planetary nebulae
to the solar system
Kwok, S
International Journal Of Astrobiology, 2009, v. 8 n. 3, p. 161-167
2009
http://hdl.handle.net/10722/90480
Creative Commons: Attribution 3.0 Hong Kong License
International Journal of Astrobiology 8 (3) : 161–167 (2009) Printed in the United Kingdom
doi:10.1017/S1473550409004492 f Cambridge University Press 2009
Delivery of Complex Organic Compounds
from Planetary Nebulae to the Solar
System
Sun Kwok
Faculty of Science, The University of Hong Kong, Pokfulam Road, Hong Kong
e-mail: [email protected]
Abstract : Infrared spectroscopic observations of planetary nebulae and proto-planetary nebulae have
shown that complex organic compounds are synthesized in these objects over periods as short as a
thousand years. These compounds are ejected into the interstellar medium and spread throughout the
Galaxy. Evidence from meteorites has shown that these stellar grains have reached the Solar System,
and may have showered the Earth during the heavy bombardment stage of the Early Earth. In this
paper, we discuss the chemical structure of stellar organic grains and compare them to the organic
matter found in meteorites, comets, asteroids, planetary satellites, and interplanetary particles. The
possibility that the early Solar System was chemically enriched by organic compounds ejected from
distant stars is presented.
Received 22 March 2009, accepted 17 April 2009, first published online 26 May 2009
Key words : chemical synthesis, infrared spectroscopy, meteorites, molecular spectroscopy, origin of life, stellar
evolution.
Introduction
Since the Miller–Urey experiment in 1953 (Miller 1953;
Miller & Urey 1959), the endogenous origin of organic matter
has remained popular for 50 years. The Miller–Urey experiment was seen by the scientific community as the experimental confirmation of the ideas of Oparin (1938) and
Haldane (1929) that origin of life on the early Earth can be
explained using only the laws of chemistry and physics.
Although the idea of panspermia has been around for some
time (Arrhenius 1908), it has never found much support except by the works of Hoyle and Wickramsinghe (see, e.g.,
Hoyle & Wickramsinghe 1977). This perception changed,
however, with the discovery of extensive presence of organic
compounds in primitive meteorites, specifically carbonaceous
chondrites (Cronin & Chang 1993), raising the possibility that
some of the organic matter on Earth may have been delivered
from extraterrestrial sources. From the crater records of the
Moon, we learn that the Earth has suffered from heavy
bombardment (Kring 2008) and a large amount of extraterrestrial carbon and other minerals was likely to have been
delivered to the Earth from comets, asteroids, and interplanetary dust particles (Chyba et al. 1990; Chyba & Sagan
1992).
The development of high-frequency radio receivers and
their application in millimetre and submillimetre-wave astronomical spectroscopy have made possible the detection
of over 140 gas-phase molecules in the interstellar medium,
the majority of which are organic molecules (Ehrenfreund &
Charnley 2000, for a complete current list, see http://aramis.
obspm.fr/mol/list-mol.html). These detections clearly show
that organic molecules of moderate complexity can be synthesized beyond the terrestrial environment, even under extreme low-density conditions. However, there is a limit to the
degree of complexity for molecules to be detected through
their rotational transitions. As the number of energy levels
increases with the complexity of the molecule, the relative
population in each of the rotational states decreases, therefore producing a weaker line. Even with increasing instrument sensitivity, there is a limit at which every part of the
millimetre and submillimetre-wave spectrum is filled with
weak lines of common abundant molecules (such as methanol), therefore crowding out lines of larger molecules and
making the detection of transitions from less abundant complex molecules difficult.
Recent advances in infrared spectroscopy, in particular
from space-based observatories, have opened up the possibility of detecting the stretching and bending modes of
complex molecules. These observations reveal the presence
of organic compounds consisting of hundreds of atoms,
with definite aromatic and aliphatic structures (Pendleton &
Allamandola 2002). Most interestingly, we now have evidence that these compounds are formed over a very short
time (y104 yr) during the late stages of stellar evolution. In
this paper, we will summarize the evidence for the synthesis
and chemical evolution of organic compounds, and discuss
their possible relationship to similar compounds found in
meteorites.
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Sun Kwok
Evolved stars as molecular factories
The discovery of large-scale mass loss in asymptotic giant
branch (AGB) stars in the 1970s led to the realization that the
majority of stars in the Milky Way Galaxy (stars with initial
masses between 1 and 8 M, corresponding to y95 % of stars
with lifetimes within the age of the Galaxy) can shed most of
their envelope mass and avoid the fate of ending their lives as
supernovae (Kwok 1987). The mass loss process takes place
in the form of a steady stellar wind over the last y106 years of
the AGB evolution, and manifests itself through line emission
from molecules in the wind, as well as continuum infrared
emission from solid-state particles condensed in the wind.
Near the end of the AGB, the ejection rate can be so large
that the solid particles in the ejecta completely shield the
stellar photosphere from the view of the observer, and these
stars can only be seen in infrared wavelengths. Depending on
whether the photospheric chemical composition has more
oxygen than carbon, the stars with O/C >1 are mostly discovered either in OH surveys (known as OH/IR stars), and
those with O/C <1 by mid-infrared surveys (known as extreme carbon stars).
As of early 2009, over 60 gas-phase molecules have been
detected in the circumstellar envelopes of AGB stars. These
include inorganics (e.g., CO, SiO, SiS, NH3, AlCl, etc), organics (C2H2, CH4, H2CO, CH3CN, etc), radicals (CN, C2H,
C3, HCO+, etc), chains (e.g., HCN, HC3N, HC5N, etc), and
rings (C3H2). These molecules are detected primarily through
millimetre/submillimetre spectroscopic observations of their
rotational transitions, with some species detected through
infrared spectroscopic of their vibrational bands in absorption against the infrared continuum.
Solid-state particles (commonly referred to as dust in the
astronomical literature) also form from direct condensation
from the gas phase. The most commonly detected solid particle is amorphous silicates (in oxygen-rich stars) and SiC
in carbon-rich stars (Kwok et al. 1997 ; Henning 2009).
A variety of refractory oxides (corundum (a-Al2O3), spinel
(MgAl2O4), rutile (TiO2)) are also present (Posch et al. 1999,
2002). For highly evolved carbon-rich stars, the dust component is believed to be dominated by amorphous carbon,
which emits a strong featureless continuum in the infrared
(Volk et al. 2000).
Synthesis of complex organics in post-AGB
evolution
The discovery of the unidentified infrared emission bands in
the planetary nebula NGC 7027 (Russell et al. 1977) and their
subsequent identification as emissions from the stretching
and bending modes of various CH and CC bonds in aromatic
compounds (Duley & Williams 1981) has led to the realization that complex organics can be synthesized in the circumstellar environment. Since planetary nebulae are a short-lived
(y20 000 yr) phenomenon and the aromatic infrared bands
(AIB) are not seen in the spectra of their progenitor AGB
stars, it suggests that these aromatic compounds must have
been synthesized in a short time under very low-density conditions.
The key to the understanding of the origin of the aromatic
compounds in planetary nebulae lies in the observations of the
transition objects between AGB stars and planetary nebulae,
commonly referred to as proto-planetary nebulae. Protoplanetary nebulae have been a missing link in the late stages
of stellar evolution as they are very difficult to detect due
to the lack of emission lines as the circumstellar material is
yet to be photoionized. Their discovery was made possible
by the Infrared Astronomical Satellite (IRAS) all-sky infrared survey. By comparing the infrared properties of AGB
stars and planetary nebulae, it is possible to infer the
infrared properties of proto-planetary nebulae and search
for their optical counterparts among the IRAS sources that
fit the criterion (Volk & Kwok 1989; Kwok 1990). The discovery of y30 proto-planetary nebulae (Kwok 1993, an
example is shown in Fig. 1) and their subsequent observations
by the Infrared Space Observatory (ISO) have led to conclusion that aromatic compounds develop shortly after the
AGB (Hrivnak et al. 2000). Most significantly, ISO spectra
of proto-planetary nebulae show evidence for aliphatic
chains attached to the aromatic rings (Fig. 2). Signatures of
the aliphatic component take the form of the 3.4 mm CH
stretching modes and 6.9 mm in-plane-bending mode of the
methyl (—CH3) and methylene (—CH2) groups, and the 8 and
12 mm broad emission plateaus from a mixture of aliphatic
side groups (Kwok et al. 2001). The presence of the 12.1, 12.4,
and 13.3 mm out-of-plane bending modes also suggests that
the aromatic units in proto-planetary nebulae are smaller
in size (with a corresponding larger number of exposed hydrogen edges) than those in planetary nebulae (Kwok et al.
1999).
Since the transition time between the end of AGB to
the planetary nebulae stage is only several thousand years
(Schönberner 1983; Kwok 2000), the synthesis of aromatic
and aliphatic compounds must occur on a time scale shorter
than this evolutionary time. It is interesting to note that
the spectral signature of acetylene (C2H2) only appears near
the end of the AGB (Fig. 3), and benzene is detected in only
one proto-planetary nebula, AFGL 618 (Cernicharo et al.
2001). We may therefore reasonably assume that aromatics
evolved from the linear hydrocarbon acetylene which folds
into a ring as benzene, which in turn attach with other rings
into aromatics.
How such a reaction can take place in the gas phase under
extremely low-density (y106 cmx3) conditions is difficult to
understand theoretically. The process may resemble that of
soot production in flames where gas-phase molecules first
nucleate into nanometre size particles and then coalesce and
grow into grains of thousands of atoms (Harris & Weiner
1985). This scenario is supported by the fact that polyacetylenes first appear in the post-AGB phase of evolution.
In Fig. 4 we show the ISO spectrum of the proto-planetary
nebula AFGL 618, where diacetylene (C4H2) and triacetylene
(C6H2) are detected in addition to acetylene. These polyacetylenes probably contribute towards the size growth as
Delivery of Complex Organic Compounds
Fig. 1. Colour composite image of HST WFPC 2 I and V band observations of IRAS 16594-4656 (Water Lily Nebula, Hrivnak et al. 1999),
a proto-planetary nebula that shows AIB emissions (Garcia-Lario et al. 1999).
200
11.3
180
12.4
160
16.8
13.4
14.2
λFλ(10 -10 erg cm -2 s-1 )
140
7.7
120
IRAS 21282+5050
8.6
100
12.1
6.2
80
60
40
7.7
6.2: sp 2 C-C stretch
8.6: sp 2 C-H in-plane bend
7.7: sp 2 C-C stretch
11.3: sp 2 C-H out-of-plane bend
12.4: sp 2 C-H out-of-plane bend
6.9
3.3
20
IRAS 07134+1005
0
-20
2
4
6
8
10
12
14
16
18
20
Wavelength (µm)
Fig. 2. ISO SWS01 spectra of the young planetary nebula IRAS 21282+5050 and the proto-planetary nebula IRAS 07134+1005, showing
various aromatic C—H and C—C stretching and bending modes at 3.3, 6.2, 7.7, 8.6, and 11.3 mm. The proto-planetary nebulae spectra are
characterized by the 12.1, 12.4, 13.3 mm out-of-plane bending mode features from small aromatic units.
163
Sun Kwok
21318+5631
600
ISO SWS01
20.1 µm
500
25.5 µm
λFλ(10 -10 erg cm -2 s-1 )
27.2 µm
400
C 2H 2
300
200
100
0
0
10
20
30
40
50
Wavelength ( µm)
Fig. 3. The 13.7 mm n5 vibrational band of acetylene is the strongest absorption feature in the ISO SW01 spectrum of the extreme carbon
stars such as IRAS 21318+5631. Extreme carbon stars are highly evolved AGB stars with such a high mass loss rate that circumstellar dust
completely obscures the stellar photosphere and the starlight is re-emitted in the infrared through dust emission. Other features in the
spectrum are the 21 and 30 mm unidentified emission features.
2000
ISO SWS06
AFGL 618
λFλ(10 -10 erg cm -2 s-1 )
164
1500
HC 5N
HC 3N
C 6H 2
1000
C 4H 2
HCN
C 2H 2
500
13
14
15
16
17
Wavelength (µm)
Fig. 4. ISO SWS06 spectrum of the proto-planetary nebula AFGL 618. In addition to the 13.7 mm acetylene (C2H2) feature, vibrational
features of diacetylene (C4H2) and triacetylene (C6H2) can also be seen.
Delivery of Complex Organic Compounds
well as the aromatization of the grains in the post-AGB phase
of evolution.
Chemical structure of the carrier of the aromatic
infrared bands
The observed spectral characteristics of proto-planetary
nebulae suggest that the chemical structure of the carrier
must consist of some aromatic rings, but probably in units of
a small number of rings. These units are connected with each
other by aliphatic bridges, which can be of diverse nature.
The overall structure is probably disorganized and not highly
structured. A possible schematic of the structure is given by
Pendleton & Allamandola (2002), although the actual structure may contain more aliphatics than they outlined. Spectral
similarities between proto-planetary nebulae and terrestrial
coal and kerogen have been pointed out by Guillois et al.
(1996) and Papoular (2001). Kerogen differs from other
amorphous hydrocarbons such as hydrogenated amorphous
carbon (HAC, Jones et al. 1990) and quenched carbonaceous
composites (QCC, Sakata et al. 1984, 1987) in that kerogen
contains impurities such as O, N, and S, whereas HAC and
QCC are pure hydrocarbons. Under circumstellar conditions,
it is natural to assume that any condensates will include other
heavy elements as N can easily substitute C as one member
of the aromatic structure. Such nitrogen heterocycles can be
clearly identified when higher spectral resolution observations are available (Hudgins et al. 2005), and their detection
would have significant biochemical implications.
Various techniques have been employed to artificially
create carbon nanoparticles. These include condensation of
carbon vapour from arc discharge between two carbon rods
in an H2 atmosphere (Mennella et al. 1996), laser pyrolysis
of gas-phase hydrocarbons (Herlin et al. 1998), and laser
ablation of graphite and subsequent condensation under high
temperatures (Jäger et al. 2008). The 1–4 nm size fullerenelike particles created by Jäger et al. (2008) show strong a
3.4 mm aliphatic —C— H stretching band feature as well as
a 6.2 mm aromatic —C—
—C— stretching feature, suggesting
the existence of aliphatic bridges between aromatic subunits.
These onion structure grains could represent the first solid
condensate in proto-planetary nebulae.
In summary, the carbonaceous compound made in the circumstellar envelopes of proto-planetary nebulae is likely to be
a solid-state material made up of islands of aromatic rings,
connected by aliphatic chains. In addition to hydrogen, functional groups (including those containing heavy elements O
and N), such as methyl (—CH3), methylene (—CH2), carbonyl
(C—
—O), aldehydic (—HCO), phenolic (OH), and amine
(NH2), can be attached to the peripherals of these aromatic
units. These structures are subjected to radiation processing
as the star evolves to the planetary nebulae stage with an
accompanying increasing amount of ultraviolet radiation
output. This could result in consolidation of the rings and
aromatization of the structure. To what extent these chemical
structures are further processed by interstellar radiation field
once they are ejected into the interstellar medium is uncertain.
Relations between circumstellar and Solar System
organic compounds
Solar System objects display many spectral features suggesting that they contain similar minerals and organic matter as
circumstellar grains. Both amorphous and crystalline silicates
have been detected in the infrared spectra of comets (Wooden
et al. 1999). It has been known for over 20 years that carbonaceous meteorites contain an insoluble macromolecular
solid component often referred to as insoluble organic matter
(IOM, Cronin et al. 1987). IOM is predominantly aromatic in
structure but contains various functional groups (Kerridge
1999; Cody & Alexander 2005) not dissimilar to the chemical
structure of the organic solids found in proto-planetary
nebulae. From the colour of asteroids, it is inferred that
they may be covered by layers of polymer-type organic compounds which are structurally similar to kerogen (Gradie &
Veverka 1980; Cruikshank 2008).
X-ray absorption near-edge structure spectroscopy and
infrared spectroscopy of interplanetary dust particles have
revealed the presence of aliphatic CH2 and CH3, carbonyl
(C—
—O) groups (Flynn et al. 2003). The 3.4 mm CH stretching
bands observed in the spectra of meteorites (Ehrenfreund
et al. 1991) is similar to those seen in proto-planetary nebulae
(Hrivnak et al. 2007). These features are also seen in the
spectra of the Saturnian satellites, Iapetus, Phoebe, and
Hyperion, by the Visible-Infrared Mapping Spectrometer on
board the Cassini spacecraft (Cruikshank et al. 2008).
From planetary nebulae to the Solar System
While it is now widely recognized that planetary nebulae
and proto-planetary nebulae are major sources of organic
matter in the Milky Way Galaxy, it has been argued that
these stellar grains will be subjected to destruction by interstellar shocks and radiation field and therefore unable to
survive the journey through the diffuse interstellar medium
(Jones et al. 1996). The discovery of pre-solar grains of
diamonds (Lewis et al. 1987), SiC (Bernatowicz et al. 1987),
corundum (Nittler et al. 1997), spinel, etc in meteorites and
the tracing of the origin of these grains to AGB stars through
isotopic analysis has led to the amazed realization that one
can actually hold a piece of star dust in our hands. Silicate
grains of circumstellar origin have also been found in interplanetary dust particles (Messenger et al. 2003) and in
meteorites (Nagashima et al. 2004; Nguyen & Zinner 2004).
The old paradigm of planets and other Solar System objects
formed from a well-mixed primordial nebula of chemically
and isotopically uniform composition and that no extrasolar
solid could survive the formation of the Solar System (Suess
1965) is therefore no longer tenable.
Although there is no corresponding strong direct evidence
of organic compounds having arrived from outside the Solar
System, support for this hypothesis can be found in the fact
that the organic compound D/H ratios in some Solar System
objects have non-solar values (Ehrenfreund et al. 2002;
Ehrenfreund et al. 2006). The anomalous isotopic ratios of
noble gases trapped in fullerenes (C60 to C400) in the Allende
165
166
Sun Kwok
and Murchison meteorites point to their extraterrestrial
origin (Becker et al. 2000). The carbon isotope ratios of
nucleobases in the Murchison meteorite also show signs
that they are interstellar (Martins et al. 2008). The elevated
ratios of 15N/14N and D/H in organic globules in the Tagish
Lake meteorite provide additional evidence that interstellar
organics are present in the current Solar System (NakamuraMessenger et al. 2006).
the early Earth environment could play a role to facilitate or
accelerate the development of life on Earth. Although the
identification of circumstellar organics has so far been limited
to hydrocarbons of aromatic and aliphatic structures, future
improvements in sensitivity and spectral resolutions of space
infrared telescopes can lead to the identification of N, P, and
O substituted compounds that are essential elements of biochemistry.
Effects of stardust on the origin of life
Acknowledgements
The above observations have shown that planetary nebulae
are capable of manufacturing complex organic compounds
over very short time scales and ejecting them in large quantities into the interstellar medium. These organic compounds
can migrate through the Milky Way Galaxy and have probably chemically enriched the early solar nebula. The subsequent mixing and formation of the Solar System have not
completely destroyed the pre-solar materials and remnants
of these organic solids are embedded in comets and other
planetismals. Through the heavy bombardment episodes in
the early days of the Solar System, these organics have
likely been delivered to Earth from comets and asteroids.
Macroscopic complex organics are much more likely than
small molecules to have survived the impact.
If kerogen-like materials indeed represent the major constitutes of extraterrestrial organics, we may ask whether such
organics played any role in the origin of life. Terrestrial
kerogen is a solid sedimentary insoluble organic material
found in the upper crust of the Earth. Since the fractions of
H, S, N, and O relative to C in kerogen are similar to those in
lipids, they are believed to have formed as the result of decay
of living organisms. Could it be the case that in the Early
Earth, the process was reversed with fragments of extraterrestrial kerogen having served as suppliers of ingredients of
the biomolecules that formed the basis of beginnings of life ?
While meteoritic IOM could lead to membrane structures
(Deamer 1997), the emergence of genetic molecules such as
DNA and RNA require nucleobases that are heterocyclic
compounds heavy in N contents. Precursors of nucleobases
such as purine and pyrimidine have large partition functions
with their populations spread thinly over many rotational
levels and therefore are difficult to detect in circumstellar
environment by present millimetre-wave technology. The
vibrational modes of N-substituted aromatic units require
high spectral resolution to be clearly identified. With the
development of future large space-based spectroscopic telescopes, we are confident that such prebiotic molecules will be
detected in time.
I thank Bernard Foing and the organizers of the ESLAB
Symposium on Cosmic Cataclysm and Life for a stimulating
meeting. This work is supported by a grant from the Research
Grants Council of the Hong Kong Special Administrative
Region, China (Project No. HKU 7020/08P).
Conclusions
The discovery that evolved AGB stars and planetary nebulae
are prolific molecular factories that are capable of rapid
synthesis of complex organic molecules and solids has cast
doubts on the completely endogenous theory of origin of life.
The injection of extraterrestrial (and pre-solar) chemicals into
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